1. Field of the Invention
The present invention relates to an actively controlled automotive suspension system which is capable of actively varying suspension characteristics. Specifically to technologies for effectively suppressing vehicular body attitude change, such as rolling, pitching, bouncing or so forth.
2. Description of the Background Art
Recently, there have been proposed and developed various active suspension systems which act in an active manner for suppressing vehicular body attitude change according to vehicle driving condition and thus achieves both of riding comfort and driving stability. Prior art active suspension systems traditionally comprises a fluid pressure operated cylinder, for example a hydraulic cylinder, interposed between a vehicle body and a suspension member, a fluid pressure control valve communicated with a pressure chamber in the cylinder for adjusting working fluid pressure in the pressure chamber, a pressure accumulator connected through a fixed throttling orifice to the pressure chamber of the cylinder, a plurality of sensors for monitoring vehicular body attitude change, and a controller for controlling the pressure control valve based on the vehicular body attitude change signal value from the sensors. Such conventional active suspension systems are constructed such that damping force created by a first fluid pressure system including the pressure control valve becomes larger than or equal to that created by a second fluid pressure system including both of the pressure accumulator and the fixed throttling orifice, when vibration input from the suspension member to the cylinder is greater than or equal to a predetermined frequency.
As is generally known, the conventional active suspension system operates in such a manner as to suppress the vehicular attitude change by actuation of the pressure control valve, when fluid pressure fluctuation in the pressure chamber of the fluid pressure operated cylinder occurs due to a relatively low frequency vibration at around a resonance frequency for a sprung mass (corresponding to the vehicular body) at which the vehicle experiences a great attitude change. In addition, the conventional active suspension system operates in such a manner as to absorb the input vibration by the pressure accumulator with damping created due to flow restriction action of the fixed throttling orifice, when the pressure fluctuation in the cylinder occurs due to a relatively high frequency vibration input at around a resonance frequency for an unsprung mass (corresponding to the suspension member). One such conventional active suspension system has been disclosed in Japanese Patent First Publication (Tokkai) Showa 62-292517.
In the previously noted conventional active suspension systems, the orifice provided between the cylinder and the accumulator has a fixed throttling characteristics. A throttling rate of the orifice is fixed and tuned effectively to damp pressure fluctuation in the pressure chamber in the cylinder, occurring due to a relatively high frequency vibration (at around a resonance frequency for the sprung mass) input from the suspension member. On the other hand, a fluid resistance of the first fluid pressure system is determined depending upon both of the input vibration frequency and a boundary frequency defining a boundary between two resonance frequencies for the sprung mass and the unsprung mass. That is, when the input vibration frequency is less than the boundary frequency, the pressure control valve is controlled such that the fluid resistance in the first fluid pressure system becomes less than that of the second fluid pressure system including the orifice so as to facilitate or promote changes in fluid pressure in the pressure chamber. When the input vibration frequency is greater than or equal to the boundary frequency, the pressure control valve is also controlled such that the fluid resistance in the first fluid pressure system exceeds that of the second fluid pressure system including the orifice so as to prevent fluid pressure fluctuation in the pressure chamber. As is well known, since a fluid flow passage area in the first fluid pressure system is essentially increased due to decreasing in the fluid resistance, a fluid flow velocity flowing through the fluid passage of the first fluid pressure system is decreased. Therefore, the conventional active suspension systems have a relatively low responsiveness with regard to the pressure control operation for the fluid pressure operated cylinder in various low frequency vibration modes (at around the resonance frequency for the sprung mass), for example rolling and pitching.